microPET imaging of glioma integrin α_vβ₃ expression using ⁶⁴Cu-labeled tetrameric RGD peptide

Integrin α_vβ₃ plays a critical role in tumor-induced angiogenesis and metastasis and has become a promising diagnostic indicator and therapeutic target for various solid tumors. Radiolabeled RGD peptides that are integrin specific can be used for noninvasive imaging of integrin expression level as well as for integrin-targeted radionuclide therapy. Methods: In this study we developed a tetrameric RGD peptide tracer ⁶⁴Cu-DOTA-E{E[c(RGDfK)]₂}₂ (DOTA is 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid) for PET imaging of integrin α_vβ₃ expression in female athymic nude mice bearing the subcutaneous UG87MG glioma xenografts. Results: The RGD tetramer showed significantly higher integrin binding affinity than the corresponding monomeric and dimeric RGD analogs, most likely due to a polyvalency effect. The radio-labeled peptide showed rapid blood clearance (0.61 ± 0.01 %ID/g at 30 min and 0.21 ± 0.01 %ID/g at 4 h after injection, respectively [%ID/g is percentage injected dose per gram]) and predominantly renal excretion. Tumor uptake was rapid and high, and the tumor washout was slow (9.93 ± 1.05 %ID/g at 30 min after injection and 4.56 ± 0.51 %ID/g at 24 h after injection). The metabolic stability of ⁵⁴Cu-DOTA-E{E[c(RGDfK)]₂}₂ was determined in mouse blood, urine, and liver and kidney homogenates at different times after tracer injection. The average fractions of intact tracer in these organs at 1 h were approximately 70%, 58%, 51%, and 26%, respectively. Noninvasive microPET studies showed significant tumor uptake and good contrast in the subcutaneous tumor-bearing mice, which agreed well with the biodistribution results. Integrin α_vβ₃ specificity was demonstrated by successful blocking of tumor uptake of ⁶⁴Cu-DOTA-E{E[c(RGDfK)] ₂}₂ in the presence of excess c(RGDyK) at 1 h after injection. The highest absorbed radiation doses determined for the human reference adult were received by the urinary bladder wall (0.262 mGy/MBq), kidneys (0.0296 mGy/MBq), and liver (0.0242 mGy/MBq). The average effective dose resulting from a single ⁶⁴Cu-DOTA-E{E[c(RGDfK)]₂} ₂ injection was estimated to be 0.0164 mSv/MBq. Conclusion: The high integrin and avidity and favorable biokinetics make ⁶⁴Cu-DOTA- E{E[c(RGDfK)]₂}₂ a promising agent for peptide receptor radionuclide imaging and therapy of integrin-positive tumors.